The present invention relates to communication systems and more particularly to a method and apparatus for communicating over electric power lines in a direction from load to source.
The use of electric power lines for signaling to convey information from a point of consumption to a point more centrally located with respect to the source of electrical energy is well known in the art. Representative examples of schemes proposed for communicating intelligence in the direction from load to source i.e. in an "inbound" direction are described in the following U.S. patent:
(1) U.S. Pat. No. 3,509,537, issued Apr. 28, 1970 to C. F. Haberly, for "Digital Communication Apparatus For A Power Distribution Line"--in which a signaling load is alternately impressed on and removed from the line at a frequency that is a subharmonic of the line frequency; the current modulation thereby produced contains frequencies not normally present in the line and the signal is sensed by selectively detecting such frequencies in the phase current at the central power station.
(2) U.S. Pat. No. 3,258,692 issued June 28, 1966 to O. J. Jacomini et al, for "Automatic Reading Apparatus For Plural Meters By Transmitted Coded Pulse Trains"--in which a passive load is or is not impressed as an additional load at the customer's premises at a 30 hertz rate according as the bit to be transmitted as a one or a zero; a 30 hertz filter is used to extract the intelligence.
(3) U.S. Pat. No. 3,733,586 issued May 15, 1973 to J. F. Lusk et al, for "Meter Interrogation System Having Strobe Logic Control"--in which frequency shift keying is used at frequencies sufficiently high that multi-path time dispersal must be considered (typically 900 Hz and 1100 Hz).
(4) U.S. Pat. No. 3,445,814 issued May 20, 1969 to A. Spaltil, for "System For Interrogating Remote Stations Via Power Lines of An Electrical Distribution Network"--in which a series-resonant circuit with time constant short relative to the period of the line excitation is shorted across the power line near the crest of the voltage wave; the polarilty of the voltage wave is opposite on successive applications and the interval between successive applications is a characteristic of the bit to be transmitted. Information is extracted at the district receiver station based on the electrically distinguishable characteristics of the individual pulses from different transmitting sites and on the intervals between successive pulses.
(5) U.S. Pat. No. 3,460,121 issued Aug. 5, 1969 to W. H. Wattenburg et al, for "Signaling and Communication System"--in which signaling is accomplished by means of a dynamic load which is impressed on the power line voltage; the nature of this dynamic load is such that during signaling the load is continuously switched on and off electronically at a frequency harmonically related to the power line frequency by means of internal control circuitry.
Additionally, recent work has been reported by Automated Technology Corporation, by General Electric Company and by Westinghouse Electric Corporation, in which communication over power distribution lines is accomplished at frequencies of the order of 100 kHz at which frequency the signals traverse the distribution transformers by virtue of the capacitance between primary and secondary windings. In these programs the scheme of Automated Technology Corporation utilizes frequency shift keying to frequencies on opposite sides of a mid frequency to represent ones and zeros, while the General Electric Company utilizes on-off keying of a central frequency.
The above mentioned approaches tend to suffer from one or more of a variety of difficulties. The high frequency schemes experience excessive attenuation on traversing distribution transformers; this attenuation depends somewhat upon the design of the transformer but can easily amount to 20 to 30 decibels. Power factor correcting capacitors tend to absorb the high frequency components and thus act detrimentally on all schemes that depend upon the transmission of frequency components appreciably above that of the power supply frequency. Schemes that employ loading at subharmonic frequencies tend to suffer from very low signal rates in that several cycles must be sensed in order to achieve a narrow bandwidth--and if the bandwidth is not narrow then the signal-to-noise ratio suffers. When narrow band receivers are required for frequencies generated by subharmonic loading of the power line voltage there arises the additional problem of ensuring that the frequency band correctly tracks the changes in frequency that are attributable to drift of the instantaneous frequency of the power system--though this problem may not arise in the case of receivers of affordable cost for low frequency reception.
It is a general object of the present invention to provide a method and apparatus for central power line communication which overcome the various disadvantages of the earlier approaches to communication over power lines in the inbound direction.
It is specific object of the present invention to provide a method for electric power line communication which utilizes a signal waveform that approaches closely the waveform for which the electrical distribution system was designed.
It is another object of the present invention to generate a signal current by impressing loads on the line voltage by means of silicon controlled rectifiers (SCR) or triacs employed as switches in a duty cycle which permits the high transient current capability of SCR's or triacs to be exploited without also causing substantial thermal dissipation.
It is a feature of the invention that high peak signal power levels can be achieved while maintaining low average power dissipation without incurring excessive costs through the use of SCR's or triacs as the switching element.
It is another feature of the present invention that the high power signaling can be accomplished without introducing noticable flicker on lighting loads on the customers premises and without interfering with the operation of television sets or other voltage sensitive apparatus.
It is a further feature of the present invention that detection is accomplished in a fashion which selectively accumulates valid signal while discriminating against all currents attributable to steady loads or transient switching.
It is still another feature of the present invention that simultaneous noninterfering inbound transmissions can be superimposed on the same phase.
It is an additional feature of the present invention that detection is accomplished in processing relatively few cycles of the current wave.
It is a further feature of the invention that the inbound transmissions occur on individual phases of the power system and accordingly simultaneous inbound transmissions on the multiple phases is achievable.
It is another feature of the invention that priority messages can be detected in the presence of other messages.